The definition
Biological evolution, simply put, is descent with modification. This definition
encompasses small-scale evolution (changes in gene frequency in a population
from one generation to the next) and large-scale evolution (the descent of
different species from a common ancestor over many generations). Evolution helps
us to understand the history of life.

The explanation
Biological evolution is not simply a matter of change over time. Lots of things
change over time: trees lose their leaves, mountain ranges rise and erode, but
they aren't examples of biological evolution because they don't involve descent
through genetic inheritance.

The central idea of biological evolution is that all life on Earth shares a common
ancestor, just as you and your cousins share a common grandmother.

Through the process of descent with modification, the common ancestor of life on Earth
gave rise to the fantastic diversity that we see documented in the fossil record and
around us today. Evolution means that we're all distant cousins: humans and oak trees,
hummingbirds and whales.